On April 5 of this year, Trans-Siberian Orchestra as fans have known it, changed forever.

That was the day Paul O’Neill, the founder of the combination progressive rock band and orchestra, passed away from an accidental reaction to prescribed medications he’d been taking. As the main songwriter and the man who developed the stories and concepts behind each album from TSO, the group’s future was altered with O’Neill’s passing.

But Al Pitrelli, who serves as musical director for one of the two TSO units that bring the band’s Christmas tours to arenas nationwide (Derek Wieland heads up the other unit), said fans won’t see TSO fade away any time soon.

Both Oliva and Pitrelli have been involved since the early days of TSO. The group was designed to carry on without O’Neill or any of its main music contributors. The group was not based around a frontman or primary instrumentalist. Instead O’Neill cast musicians, and especially singers, to fit whatever song he was writing.

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Finding the right voice for certain songs proved difficult at times – the album “Night Castle” was delayed for four years until O’Neill found singer Jeff Scott Soto to voice a key character, Lt. Cozier. Still, the idea was anyone was replaceable, and at the very least, TSO would be able to continue doing its annual Christmas tours – easily the biggest and most popular holiday tour that goes out each season – for many years to come.

What isn’t so certain is the future of several album projects that were in progress when O’Neill passed away. Pitrelli expects that with Oliva and O’Neill’s wife, Desiree, involved, these projects will come to fruition.

“Paul and his writing partner, Jon Oliva, have quite a few projects in the works. There are a few records that we had been recording. Maybe we’re halfway done, 60 percent done, things like that,” Pitrelli said, mentioning “Romanoff: When Kings Must Whisper,” “Streets: A Rock Opera” and “Gutter Ballet,” as three rock operas that are in progress. “There’s so many things we were working on during the downtime in between tours. But Paul and his family, Paul and his wife, this (TSO) was their child that they gave birth to years and years and years ago. It’s so nice to know that the family is going to carry on the family’s legacy.

“When you have a family with this much depth just don’t be surprised what happens later on,” he said. “It’s going to be an amazing journey.”

What TSO accomplished during O’Neill’s life was already impressive and quite unique within the music industry.

The seeds of the project started to sprout when O’Neill came on board to produce the rock band Savatage, which at the time included Oliva, Pitrelli and Robert Kinkel.

O’Neill already had the idea of combining a rock band and orchestra to make rock operas, with many of the stories drawn from historical figures and events. Atlantic Records bought into O’Neill’s vision and gave him the finances and creative freedom to turn his ideas into reality. And O’Neill brought along Oliva, Pitrelli and Kinkel to form his creative team.

O’Neill first hit paydirt by breaking into the Christmas market with the elaborate albums “Christmas Eve and Other Stories” in 1996, followed by “The Christmas Attic” in 1998. The first album eventually became a triple-platinum hit (with the track “Christmas Eve Sarajevo,” hitting the top 50 on “Billboard” magazine’s Hot 100 singles chart three straight holidays as it accumulated 1.3 million downloads), while “The Christmas Attic” has gone double platinum. The trilogy was completed with the double-platinum “The Ghosts of Christmas Eve” in 2004.

In 1999, TSO did its first holiday tour, playing arenas and treating fans not only to its music, but delivering an innovative, eye-popping show filled with pyrotechnics, lighting effects and other visual bells and whistles that almost made a Kiss concert seem stark by comparison.

Along the way, O’Neill and company ventured into non-Christmas rock operas, the first of which was “Beethoven’s Last Night.” It’s since been followed “Night Castle” in 2009 and “Letters from the Labyrinth” in 2015.

This year’s concert will be anchored by a performance of the compilation album “The Ghosts of Christmas Eve: The Best of TSO and More.” Considering that many of the performers and production people working on this year’s tour have been involved for many years, fans can expect the live show to deliver the musical favorites and visual spectacle that has always defined the holiday tour and kept fans filling arenas year after year.

“We’ve become such a tradition. We’ve become to people what “It’s a Wonderful Life” or “Miracle on 34th Street” was to me when I was kid,” Pitrelli said. “This is something that people who have latched onto and made part of their holidays. Their families enjoy it and there are multi-generations of families just coming out and watching it. It’s a lot of fun…When you’re doing “The Ghost of Christmas Eve” like we are again this year, it’s almost the greatest hits collection anyway so everybody will hear their favorites.”

Trans-Siberian Orchestra presents The Ghosts of Christmas Eve

When: 8 p.m. Dec. 8

Where: Intrust Bank Arena, 500 E. Waterman St.

Tickets: $48-$78 available at selectaseat.com, by phone at 855-755-7328 or at the arena box office.